This invention relates to electrical switching apparatus, and more particularly to an actuator device for an electric switch for providing high contact operating speed without regard to the speed at which the operating handle is rotated.
Contact arcing is a significant problem in high-current electric switches. Arcing may occur both during the process of closing a switch and during the process of opening the switch. A typical switch has at least one moving contact and one fixed contact. In the open position, the moving contact is in a position far removed from the fixed contact, and an insulating medium (e.g. gas, oil or a vacuum) separates the contacts. If power has not been disabled at another point in the circuit, there will typically be a difference in electrical potential between the moving and fixed contacts, but the contacts are separated by sufficient distance to establish an ionization potential which exceeds the applied voltage and no current may flow therebetween. In order to close the switch, or establish an electrical connection between the moving and fixed contacts, the moving contact is brought into mechanical and electrical engagement with the fixed contact. As the moving contact approaches the fixed contact, and the distance-sensitive ionization potential falls below the applied voltage, an arc occurs. The arc continues until the moving contact reaches the fixed contact, effectively short-circuiting the arc.
When opening a switch that controls a circuit in which current is flowing, the moving contact is removed from the fixed contact. The initial distance between the contacts is extremely small, and the potential difference between the contacts exceeds the ionization potential, again producing an arc. Once an ionized path for current is produced, the arc may continue until the path is disturbed, with the result that an arc may remain present even after the contacts are separated by a large distance.
Arcs are particularly dangerous, because if a circuit controlled by a switch is faulty, the arc may carry the entire fault current, producing extremely large amounts of heat, concentrated in a small region of the switch tank. Even when conducting lesser values of current attributed to its normal load, failure to limit the arc to a relatively short period will concentrate excessive thermal energy within this region. This may cause severe damage to the switch, and may even cause the switch to explode. It is therefore highly desirable to minimize arcing.
One solution to the arcing problem is to move the contacts extremely rapidly during opening and closing operations. Rapid contact movement minimizes the time during which the contacts are close enough for an arc to strike or be maintained. Unfortunately, human operators do not always apply (and may not be capable of applying) sufficient rapid force to the switch operating handle to achieve satisfactory contact movement speeds.
Accordingly, manufacturers of electric switching apparatus have developed actuator devices which move the contacts extremely rapidly during opening and closing operations. These devices typically receive energy supplied by user operation of an operator handle, store the energy temporarily until a predefined stored energy threshold is reached, and release the energy rapidly to drive the contacts at extremely high speed. Reference is made to U.S. Pat. Nos. 3,590,183, 4,412,116, and 4,554,420, and to McGraw-Edison Power Systems Division Catalog, Section 260-30, Page 9 (July 1971) which disclose representative known switch actuator devices.
Known switch actuators present a number of significant disadvantages. One type of prior-art actuator is known as an "over-toggle" actuator. A pivot lever is operatively connected to the contact operating shaft. The pivot lever is mounted for limited rotation about a pivot axis. One end of a resilient spring is attached to the lever and the other end of the spring is anchored to a pivotable reaction piece on the opposite side of the pivot axis. The spring may be charged in tension or compression. The spring is least charged when the lever is at the beginning of the lever's rotational range, and is most heavily charged when the lever is near the extreme of its operational range. Thus, when the lever is in any position other than at either end of its rotational range, the spring applies pressure to return the lever to one of the ends.
Typical devices of the "over-toggle" variety lack positive latching arrangements that prevent forces developed in the switch from forcing the operator shaft away from the desired position during operation. In addition, the mechanical arrangements these devices generally use are not as volumetrically efficient in transferring energy as may be desired. For a given operating force supplied by a user, these devices typically do not store sufficient energy to produce the desired rotational output, or devices with sufficient energy require undesirably large amounts of space to house the spring and energy transferring mechanism. Some devices have been developed using torsion springs to eliminate one or more of the above disadvantages, but such devices have often utilized extremely complex and expensive torsion spring designs.
In addition to the above problems, prior art "over-toggle" type actuators suffer from tolerance problems in that the precise point at which the switch contacts are released, with respect to the position of the operator handle, is often less accurately controlled. Also, the output from these actuators rotates in an opposite direction from the input. This may be confusing for switch users. In addition, the means for coupling energy or torque into and out of the actuator have typically been located outside the boundaries of the actuator. As a result, a larger volume is required to house the actuator. In addition, the means for supplying torque into the actuator typically does not include flexibility to permit angular or parallel offsets of the input shaft.